The invention relates generally to video image management systems and more particularly to an improved VTR editing system.
A Video Tape Recorder (VTR) is a mechanical device that records a video (television) signal on magnetic tape and plays it back at a later time. There are other devices that perform similar functions with different technology, such as Video Disk Recorders. Such other devices record on magnetic tape, optical disk, or other media. They may record analog or digital signals. For ease of discussion, we will refer to this entire class of devices as VTRs, and call the storage medium used video tape.
VTRs usually conform to a set of national standards. In the United States, the video signal is most commonly compatible with the NTSC (National Television System Committee) standard. This specifies that the display be updated at a rate of 29.97 frames per second (fps). It specifies 525 horizontal lines of video information each frame, of which approximately 486 lines are visible on the display. The frequency allows approximately 760 visible changes in intensity in a single horizontal scan line, although in practice the usable value is between 400 and 600. In summary, NTSC video is approximately 30 fps, with a spatial resolution of 512H.times.486V picture elements (pixels). Other countries use other video standards, notable PAL (phase alternating-line) and SECAM (sequential color with memory).
VTRs often have multiple recording channels. They can record a video signal, one or more audio signals, and often use a channel to record addressing information. The VTR can modify the contents of one channel without disturbing the contents of the others. The most common format for addressing information in the United States is SMPTE (Society of Motion Picture and Television Engineers) Time Code. It states that each frame of video can be identified by a time expressed in Hours:Minutes:Seconds.Frames. Valid addresses range from 00:00:00.00 to 23:59:59.29. Each frame has 80 binary digits (bits) of information recorded on the time code channel. There are other time code standards, but SMPTE Time Code will be referred to for ease of discussion.
The important difference between a VTR and a Video Cassette Recorder (VCR), at least for the purposes of this disclosure, is that a VTR can modify a single frame of video information without changing the preceding or following frames. This action, called a "single frame edit", allows the creation of computer generated animation.